The role of excitation systems as a potential source of damping of power system electromechanical oscillations has been well established. The need for this source of damping as a stability aid has increased with the expansion of system interconnections and with the increasing trend towards more economical generator, i.e. higher reactance, designs and greater transmission loadings.
One problem associated with the development of optimum stabilizer designs for synchronous machines is that it is necessary to tailor the design stabilizer hardware to each individual excitation system.
In the supplementary stabilizing of synchronous machines through excitation controls, from an analytical point of view, the ideal variable to be used for derivation of a stabilizing signal is rotor speed. However, the difficulty of measuring rotor speed as well as the need to process such measurements through considerable lead functions to overcome the lags in the excitation system makes it more desirable to derive the stabilizing action from acceleration or accelerating power.
True accelerating power requires measurements of electrical as well as mechanical power. Recognizing that mechanical power is difficult to measure and that it generally does not exhibit variations in the bandwidth of significance for power angle oscillations, several successful applications of supplementary stabilizing have been made with the signal derived from electrical power rather than accelerating power.
Nevertheless, where substantial action can exist from governing, fast valving, manual or automatic generation control, a signal derived from electrical power alone can present problems. For instance, during rapid generation changes, through prime mover control action, a stabilizing signal derived from electrical rather than accelerating power will cause a temporary depression in voltage during periods of increases in generation. These effects make it desirable to use accelerating rather than electrical power for the supplementary stabilizing signal.
Several schemes have been proposed to measure accelerating power by deriving mechanical power through measurements of prime mover system variables, such as gate positions in the case of hydro units and steam pressures in the case of steam units. However, such schemes are impractical and require complex apparatus wherein masurement procedures are difficult.
In a paper entitled "Practical Approaches to Supplementary Stabilizing From Accelerating Power" by F. P. de Mello, L. N. Hannett and J. M. Undrill presented at the IEEE Power Engineering Society Summer Meeting in Mexico City, Mexico on July 17-22, 1977, IEEE Trans. paper F 77-524-2 (7 pages), there is proposed an approach for deriving mechanical power purely from the measurement of electrical quantities with a stabilizing action approximating that which would be ideally realizable from measured accelerating power. This approach derives all necessary information from terminal voltage and current of a generating unit as available from potential and current transformers. The method employs a watt transducer measuring electrical power output and a frequency transducer connected to measure the frequency of a voltage synthesized from terminal voltage and current to emulate a voltage whose phase angle follows the machine rotor angle. The stabilizing action is introduced as a modulation of the voltage feedback to a voltage regulator. This same approach for stabilizing of synchronous machines was previously set forth by the same authors of such IEEE article in a proposal by Power Technologies, Inc., the assignee of the present patent application, in December 1976 in the Company Bulletin PTI/84 (5 pages) entitled "System Stabilizer STB/1", wherein a general conceptual approach is described with the view to subsequent development of specific systems for investigating the described methods. The watt and frequency transducers, operational amplifiers and SCR modulator are proposed in such papers to form the accelerating power signal which is used to modulate the terminal voltage feedback to the voltage regulator.
In practically all applications of supplementary stabilization through excitation control, implementation has been with solid state analog components which receive the input signal from transducers and produce an analog signal fed into the reference of the automatic voltage regulator. In some cases, transducers are required and there is the possibility of interactions with shaft torsional modes. Also, stabilization signal adjustments are carried out with analog signals which are subject to drift or calibration problems. Such analog signal stabilizing action often involves changing the gain in the stabilizing network as a function of load level which is subject to inaccuracies and slower response times.
U.S. Pat. No. 3,474,323 to L. A. Kilgore et al discloses a stabilizing control means which is responsive to instantaneous changes in the real power output of the machine due to transient system disturbances and provides a signal which opposes and substantially cancels the action of the voltage regulator during such transient conditions. A firing circuit provides a phase modulated signal to produce an average AC potential and excitation current as determined by an error signal. The average excitation current maintains the output voltage of the generator at the desired regulated magnitude.
Various types of voltage regulating devices are known wherein digital control means are employed for switching the taps on a transformer. For example, U.S. Pat. No. 3,818,321 to Willner et al discloses a voltage regulator wherein a digital counter is used to control the selection of the tap-switch position on an autotransformer. Similarly, U.S. Pat. No. 4,105,964 to Lebedev et al discloses the use of an electronic commutator to control transformer tap selection in a voltage regulation and stabilization device, and the U.S. Pat. Nos. 3,515,980 to Throop and 3,898,568 to Barth disclose the use of counting and switching logic means for changing the taps on a transformer or an autotransformer. It would be desirable to develop a stabilizing system using digital switching and logic for synchronous machines without the disadvantages heretofore associated with such stabilizing systems.
It is an object of the present invention to provide a system for stabilizing a synchronous machine which does not employ analog components and transducers and does not operate with analog signals fed to the automatic voltage regulator of the machine. It is another object to provide a power system stabilizer which uses the modulation with appropriate response characteristics of the voltage feedback to an automatic voltage regulator preserving the normal signal level from potential transformers and therefore allowing its use on any type and vintage of voltage regulator. It is another object to provide a system for stabilizing a synchronous generator which does not employ analog measuring techniques or transducers and which eliminates the possibility of interactions with shaft torsional modes. It is another object to provide a system for stabilizing a synchronous machine wherein the hardware normally used to measure frequency from pulse counts between zero crossings is eliminated.